The invention relates to a method of manufacturing a structure comprising at least one tube and at least one plate of a metal-matrix composite material, the tube and the plate being assembled to each other.
Such a method can be used in particular for manufacturing structures for use in the space industry, such as trellis structures used for supporting on-board optical instruments of the space telescope type, etc.
When optical instruments such as space telescopes are mounted on board space vehicles, the instruments are generally mounted on trellis structures.
As a general rule, trellis structures comprise one or more mutually parallel plates and a plurality of tubes interconnecting the plates in various directions so as to provide an assembly having a high level of dimensional stability.
In addition to these requirements for stability, the tubes and the plates constituting trellis structures that support optical instruments fitted to space vehicles need to present appropriate mechanical characteristics together with mass that is as low as possible. This often leads designers of such space vehicles to use metal-matrix composite materials for making the tubes and the plates of trellis structures.
More precisely, the tubes and the plates are usually made separately, by injecting a metal alloy into a mold containing a preform of woven fibers. The tubes and the plates are then assembled together using various techniques such as adhesive, screw fastening, or brazing.
Nevertheless, those various assembly techniques all suffer from significant drawbacks.
Thus, when the tubes and the plates are assembled together by means of adhesive, the different material inserted in this way between the pieces constitutes a source of thermo-mechanical instability, in particular because of the poor stiffness and the affinity for water of adhesive. In addition, that type of assembly gives rise to difficulties of implementation associated, for example, with the needs for surface treatment and for the adhesive to have time to polymerize.
When the tubes and the plates are assembled together by screw fastening, thermo-mechanical instability is also imparted because of friction and the micro-slippage that occurs between the pieces. In addition, adding screws and washers increases the overall mass of the structure which is undesirable for use in space.
Finally, when the tubes and the plates are assembled together by brazing, difficulties of implementation are encountered because of the respectively cylindrical and plane shapes of the pieces to be assembled together.
As illustrated in document EP-A-0 610 620, it is known to assemble two steel tubes together in a T-configuration by molding an aluminum or aluminum alloy shell around the junction zone between the two tubes.
A particular object of the invention is to provide a method of manufacturing a structure comprising at least one tube and at least one plate of metal-matrix composite material, while avoiding the drawbacks of existing methods and making it possible, in particular, to avoid imparting any thermo-mechanical instability to the junction between the tube and the plate.
According to the invention, this result is achieved by means of a method of manufacturing a structure comprising at least one tube and at least one plate of metal-matrix composite material that are connected to each other, the method comprising the following steps in succession:
placing a first fiber preform having substantially the shape of the tube, a second fiber preform having substantially the shape of the plate, and a third fiber preform surrounding an end of the first fiber preform adjacent to the second fiber preform in a cavity of a single mold; and
injecting a metal or a metal alloy into the cavity of the mold.
Implementing this method makes it possible to avoid having interfaces between the tubes and the plates, where such interfaces are to be found in all of the prior art techniques. This preserves the thermo-mechanical stability of the structure. In addition, there is a saving in mass compared with the technique of assembly by means of screw fastening, and there is a simplification of implementation compared with the technique of assembly by means of adhesive. In addition, making a plurality of identical pieces using the same mold gives rise to a reduction in cost.
In the commonest case where the tube is hollow, a mandrel is also placed in the mold inside the first fiber preform, the shape of the mandrel being complementary to the inside of the tube.
In a preferred implementation of the invention, the third fiber preform is made in the form of two half-shells, each having the shape of a half-ring.
Advantageously, the metal or metal alloy is injected into the cavity of the mold substantially along a longitudinal axis of the first fiber preform via a face of the second fiber preform remote from the first fiber preform.
In addition, and preferably, the first, second, and third fiber preforms are made by draping so that some of the fibers in each of the fiber preforms are substantially in alignment when the preforms are placed in the mold.